Radio frequency (RF) receivers are used in a wide variety of applications such as television, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, radios and other devices that receive RF signals. RF receivers all require frequency translation or mixing. For example, a television receiver may translate one channel in the band of 48 MHz to 870 MHz to an intermediate frequency of 44 MHz. And within the United States, FM radios will typically translate FM audio signals, which are broadcast in 200 KHz channels in the frequency band from 88.1 MHz to 107.9 MHz, to an intermediate frequency of 10.7 MHz.
The majority of today's RF receivers perform the necessary frequency translation or mixing using an oscillator and analog multiplier. FIG. 1 illustrates in schematic form a local oscillator and mixer circuit 60 known in the prior art that uses this technique. Circuit 60 includes an oscillator 62 and a mixer 64. Oscillator 62 forms local oscillator signal labeled “LO” in the form of a sine wave having a frequency of fLO. Mixer 64 mixes the RF input signal having desired spectral content at fIN with the local oscillator signal to form an output voltage signal labeled “VOUT” having spectral content at frequencies equal to the sum and difference of the input frequencies, namely fIN+fLO and fIN−fLO.
Mathematically, the RF input signal can be expressed as:RF=AIN(cosωINt)  [1]where AIN is the amplitude of the RF input signal, and ωIN is the radian frequency. Similarly the local oscillator signal can be expressed as:LO=ALO(cosωLOt)  [2]where ALO is the amplitude of the local oscillator signal, and ωLO is the radian frequency. The product formed at the output of the mixer can be expressed as:
                              V          OUT                =                                            [                                                A                  IN                                ⁡                                  (                                                            cos                                              ω                        IN                                                              ⁢                    t                                    )                                            ]                        ⁡                          [                                                A                  LO                                ⁡                                  (                                      cos                    ⁢                                                                                                                     ω                          LO                                                                    ⁢                      t                                                        )                                            ]                                ⁢                                          ⁢                                          =                                                                      A                  IN                                ⁢                                  A                  LO                                            2                        ⁡                          [                                                                    cos                    ⁡                                          (                                                                                                                               ω                            IN                                                                          ⁢                                                  -                                                                                                                 ω                              LO                                                                                                                          )                                                        ⁢                  t                                +                                                      cos                    ⁡                                          (                                                                                                                               ω                            IN                                                                          ⁢                                                  +                                                                                                                 ω                              LO                                                                                                                          )                                                        ⁢                  t                                            ]                                                          [        3        ]            One of these components forms the channel spectrum translated to the desired frequency and the other component can be filtered out. Oscillator 62 can be implemented, for example, by a tuned inductor-capacitor (LC) oscillator, a charge relaxation oscillator, or a ring oscillator.
An alternative is shown in FIG. 2, which illustrates in partial block diagram and partial schematic form another local oscillator and mixer circuit 80 known in the prior art. In circuit 80 a digitally synthesized oscillator 82, also known as a direct digital frequency synthesizer (DDFS), is used to generate the LO signal. The output of DDFS 82 is converted into an analog signal using a digital-to-analog converter (DAC) 84 for input to a mixer 86. This technique of generating the local oscillator signal has several advantages compared to the analog oscillator used in FIG. 1, including wide tuning range, high noise immunity, minimal self-mixing, and minimal leakage. However, the Nyquist criterion that forces DDFS 82 to be clocked at greater than twice the highest oscillation frequency has so far limited its use to low-frequency applications.
It would be desirable to have circuitry for use in an RF receiver that is suitable for higher frequency applications. Such circuitry and related methods are provided by the present invention, whose features and characteristics will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.